Firstly peel the potatoes with a potato peeler and slice them into thin rounds and place them in a clean tea towel, rub them to get the potato slices as dry as possible. Next heat 4 tablespoons of the rapeseed oil, in a non stick frying pan (approximately 23cm in diameter), add the chorizo and onion and fry gently to release the oils from the chorizo. Cook for 2-3 minutes then add the potato slices and season well with sea salt and black pepper. Turn the heat down, cover the pan with a lid and let the mixture cook slowly for 20 minutes, shaking the pan well from time to time. The mixture is not supposed to be brown, but gently stew in the oil.
Meanwhile whisk the eggs lightly in a large bowl, do not over beat them, and season with sea salt and black pepper and add the peas. When the potato mixture is cooked, quickly transfer them to the eggs in the bowl. Put the pan back on the heat, add the remaining 2 tablespoons of rapeseed oil and turn the heat to medium. Mix the potato and egg mixture well before pouring it all back into the pan and turn the heat to its lowest setting, this will take 20 minutes to cook. Every now and then draw the edge in gently with a pallet knife. When the mixture is almost set on the top of the tortilla, remove from the heat and place a plate over the pan and invert both, so that the tortilla is on the plate.
Ease the tortilla back into the pan and cook for a further 2 minutes. Turn off the heat, leave for a few minutes in the pan to set. Turn the tortilla out of the pan onto a wooden board or plate and serve hot or cold. Cut into wedges and serve with the tomato salsa.